1. Field of the Invention
The present invention relates to an LED having vertical topology, and more particularly, to an LED having vertical topology that is capable of achieving high luminous efficiency and reliability thereof, and is also capable of improving mass productivity, and a method of making the same.
2. Discussion of the Related Art
Light Emitting Diodes (LEDs) are well-known semiconductor light-emitting devices that convert electric current into light. They have been used as light sources for display image of electronic equipment including information communication equipment, since the commercialization of red LEDs using GaAsP semiconductors in 1962, and in conjunction with green GaP:N LEDs that became commercialized thereafter.
The wavelength of light emitted by such LEDs is dependent upon the kinds of the semiconductor materials that are used to manufacture of the LEDs. This is because the wavelength of emitted light is dependent upon the band-gap of the semiconductor materials that represent an energy difference between valence-band electrons and conduction band electrons.
Gallium nitride (GaN) has a high thermal stability and a broad band gap (ranging from 0.8 to 6.2 eV), and therefore has received a great deal of attention in the development of high-power output electronic component devices and LEDs.
In LEDs, another reason why gallium nitride has attracted a great deal of interest is because it is possible to fabricate semiconductor layers emitting green, blue and white light, by using GaN in combination with other elements such as indium (In), aluminum (Al) and the like.
Due to the capability of controlling an emission wavelength via the use of GaN, the emission wavelength may be adjusted based on the intrinsic properties of the materials used to comply with the characteristics of specific equipment. For example, the use of GaN makes it possible to manufacture blue LEDs, which are beneficial for optical writing, and white LEDs, which are capable of replacing incandescent lamps.
Due to various advantages of such GaN-based materials, the GaN-based LED market is rapidly growing. As a result, technologies of GaN-based optoelectronic devices have been rapidly advancing since its commercial introduction in 1994.
As such, the fabrication technology of LEDs utilizing Group III/V nitride semiconductor materials has achieved rapid development since the mid 1990's. In particular, owing to further advanced understanding of the growth methods and structures of the nitride semiconductor materials, significant improvements have been achieved in LED's characteristics such as brightness, output, driving voltage, and electrostatic properties, as well as reliability.
Despite the rapid advancement in technologies of GaN-based semiconductor devices, the fabrication of GaN-based devices suffers from a great disadvantage in terms of high-production costs. This disadvantage is closely related to difficulties associated with growing GaN epitaxial layers and subsequent cutting of finished GaN-based devices.
The GaN-based devices are generally fabricated on a sapphire substrate. This is because the sapphire substrate is commercially available in a size suited for the mass production of the GaN-based devices, supports a relatively high quality GaN epitaxial growth, and exhibits high processability in a wide range of temperatures.
Further, the sapphire substrate is chemically and thermally stable, and has a high-melting point, thus making it possible to perform a high-temperature manufacturing process, and has a high binding energy (122.4 Kcal/mole) as well as a high dielectric constant. In terms of its chemical structure, sapphire is crystalline aluminum oxide (Al2O3).
Meanwhile, because sapphire is an insulating material, the use of sapphire substrate (or any other insulating substrate) practically limits the LED device using the insulating substrate to a lateral structure.
In the lateral structure, all metal contacts used in injecting electric current into the LED are positioned on the top surface of the device structure (or on the same plane as the substrate).
In addition, as an available fabrication process of the LED device, a flip chip bonding method is widely employed that involves fabrication of an LED chip and reverse attachment of the resulting chip to a sub-mount such as a silicon wafer or ceramic substrate having excellent thermal conductivity.
However, the LED having the lateral structure or the LED fabricated using the flip chip method suffers from problems associated with poor heat release efficiency because the sapphire substrate has heat conductivity of about 27 W/mK, thus leading to very high heat resistance. Further, the flip chip method has additional disadvantages of requiring large numbers of photolithography process steps, thus resulting in complicated manufacturing processes.
On the other hand, the vertical structure is characterized in that the sapphire substrate is removed by the so-called laser lift-off (LLO) process, followed by fabrication of electrodes. In the vertical structure, one metal contact is positioned on the top surface and the other contact is positioned on the bottom surface of the device structure after removal of the sapphire (insulating) substrate.
Even though the laser lift-off process has advantages of remarkably reducing the number of the manufacturing process steps and providing excellent luminescence properties, such a conventional laser lift-off process damages the crystal structure of the LED due to thermal stress occurring between the sapphire substrate and LED structure when performing laser irradiation.
Further, nitrogen (N2) gas, separated and discharged from Ga upon performing laser irradiation, passes through the LED structure, which leads to damage of the LED crystal structure, and thus significantly reduces production yield and consequently makes it difficult to realize mass production.